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* [Bug c/100124] New: Why is "flexible array member '...' in an otherwise empty '...'" an issue?
@ 2021-04-16 20:18 gcc at behdad dot org
2021-04-16 20:50 ` [Bug c++/100124] " pinskia at gcc dot gnu.org
` (8 more replies)
0 siblings, 9 replies; 10+ messages in thread
From: gcc at behdad dot org @ 2021-04-16 20:18 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100124
Bug ID: 100124
Summary: Why is "flexible array member '...' in an otherwise
empty '...'" an issue?
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: gcc at behdad dot org
Target Milestone: ---
Hi,
In HarfBuzz we extensively use the Struct Hack [0]. Up until now we've used
"array[1]" for that. This upsets ubsan [1]. So I'm looking to replace it with
flexible arrays on compilers that support it.
However, trying to do that I get error if the flexible-array is the only
member. Clang has no issues with it, but gcc gives me: "flexible array member
'...' in an otherwise empty '...'".
```
struct UnsignedArray {
int array[];
};
```
I've seen:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69550
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71742
which suggests that disallowing flexible arrays in structs that have no other
named member is intentional. But I cannot find the reason. Can someone please
help me understand, and suggest a solution here?
Note that if I use `int array[0]` instead, then gcc warns everytime in code we
access that array with a constexpr value, like `array[0]` when we know it's
safe to do. That is, gcc seems to treat `int array[0]` literally, not as a
flexible array.
Thank you.
[0] http://c-faq.com/struct/structhack.html
[1] https://github.com/harfbuzz/harfbuzz/issues/2953
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/100124] Why is "flexible array member '...' in an otherwise empty '...'" an issue?
2021-04-16 20:18 [Bug c/100124] New: Why is "flexible array member '...' in an otherwise empty '...'" an issue? gcc at behdad dot org
@ 2021-04-16 20:50 ` pinskia at gcc dot gnu.org
2021-04-16 21:02 ` jakub at gcc dot gnu.org
` (7 subsequent siblings)
8 siblings, 0 replies; 10+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-04-16 20:50 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100124
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I will let someone comment on the flexible array extension.
But I will note GCC treats (all) arrays at the end of a POD struct as a
flexible one so the question I have is more about the ubsan issue you are
running into, is that with GCC or with clang/LLVM?
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/100124] Why is "flexible array member '...' in an otherwise empty '...'" an issue?
2021-04-16 20:18 [Bug c/100124] New: Why is "flexible array member '...' in an otherwise empty '...'" an issue? gcc at behdad dot org
2021-04-16 20:50 ` [Bug c++/100124] " pinskia at gcc dot gnu.org
@ 2021-04-16 21:02 ` jakub at gcc dot gnu.org
2021-04-17 0:03 ` msebor at gcc dot gnu.org
` (6 subsequent siblings)
8 siblings, 0 replies; 10+ messages in thread
From: jakub at gcc dot gnu.org @ 2021-04-16 21:02 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100124
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jakub at gcc dot gnu.org
--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
ISO C99 clearly says so:
6.7.2.1/16:
"As a special case, the last element of a structure with more than one named
member may have an incomplete array type; this is called a flexible array
member."
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/100124] Why is "flexible array member '...' in an otherwise empty '...'" an issue?
2021-04-16 20:18 [Bug c/100124] New: Why is "flexible array member '...' in an otherwise empty '...'" an issue? gcc at behdad dot org
2021-04-16 20:50 ` [Bug c++/100124] " pinskia at gcc dot gnu.org
2021-04-16 21:02 ` jakub at gcc dot gnu.org
@ 2021-04-17 0:03 ` msebor at gcc dot gnu.org
2021-04-17 0:51 ` pinskia at gcc dot gnu.org
` (5 subsequent siblings)
8 siblings, 0 replies; 10+ messages in thread
From: msebor at gcc dot gnu.org @ 2021-04-17 0:03 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100124
Martin Sebor <msebor at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |msebor at gcc dot gnu.org
--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
The reason why G++ rejects structs with a flexible array as the sole member is
because GCC (in C mode) doesn't accept them. Git log indicates that GCC
started rejecting such code in r38705, which was committed as part of array
member initialization cleanup. I don't know why the author of the change chose
to reject such structs rather than accepting them with a warning, just like
empty structs are accepted. If he had, C++ might have followed suit. At this
point, I don't see GCC (in either mode) making a change unless C itself were to
change first. That seems highly unlikely to me (the last proposal to relax the
rules for flexible array members didn't go anywhere).
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/100124] Why is "flexible array member '...' in an otherwise empty '...'" an issue?
2021-04-16 20:18 [Bug c/100124] New: Why is "flexible array member '...' in an otherwise empty '...'" an issue? gcc at behdad dot org
` (2 preceding siblings ...)
2021-04-17 0:03 ` msebor at gcc dot gnu.org
@ 2021-04-17 0:51 ` pinskia at gcc dot gnu.org
2021-04-17 21:13 ` msebor at gcc dot gnu.org
` (4 subsequent siblings)
8 siblings, 0 replies; 10+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-04-17 0:51 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100124
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Martin Sebor from comment #3)
> The reason why G++ rejects structs with a flexible array as the sole member
> is because GCC (in C mode) doesn't accept them. Git log indicates that GCC
> started rejecting such code in r38705
Note this change was done long before clang was around so it would be
interesting to know why clang chose differently.
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/100124] Why is "flexible array member '...' in an otherwise empty '...'" an issue?
2021-04-16 20:18 [Bug c/100124] New: Why is "flexible array member '...' in an otherwise empty '...'" an issue? gcc at behdad dot org
` (3 preceding siblings ...)
2021-04-17 0:51 ` pinskia at gcc dot gnu.org
@ 2021-04-17 21:13 ` msebor at gcc dot gnu.org
2021-04-20 19:32 ` gcc at behdad dot org
` (3 subsequent siblings)
8 siblings, 0 replies; 10+ messages in thread
From: msebor at gcc dot gnu.org @ 2021-04-17 21:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100124
Martin Sebor <msebor at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Blocks| |69698
--- Comment #5 from Martin Sebor <msebor at gcc dot gnu.org> ---
G++ accepted structs where a flexible array is the sole member until version 6.
G++ 6 tightened up the rules (in r231665) to more closely match the C front
end. I suspect Clang implemented the more permissive historical G++ behavior
in C++ mode for compatibility with G++ and the stricter GCC behavior in C mode,
and to this day hasn't changed. My impression is that the historical G++
permissiveness was due to oversights rather than deliberate features (we're
tracking a few such outstanding issues in pr69698 that are yet to be fixed).
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69698
[Bug 69698] [meta-bug] flexible array members
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/100124] Why is "flexible array member '...' in an otherwise empty '...'" an issue?
2021-04-16 20:18 [Bug c/100124] New: Why is "flexible array member '...' in an otherwise empty '...'" an issue? gcc at behdad dot org
` (4 preceding siblings ...)
2021-04-17 21:13 ` msebor at gcc dot gnu.org
@ 2021-04-20 19:32 ` gcc at behdad dot org
2021-04-20 20:30 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
8 siblings, 0 replies; 10+ messages in thread
From: gcc at behdad dot org @ 2021-04-20 19:32 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100124
--- Comment #6 from Behdad Esfahbod <gcc at behdad dot org> ---
Thank you all. I understand it's unlikely to happen at this point.
In reply to Andrew Pinski from comment #1)
> I will let someone comment on the flexible array extension.
>
> But I will note GCC treats (all) arrays at the end of a POD struct as a
> flexible one so the question I have is more about the ubsan issue you are
> running into, is that with GCC or with clang/LLVM?
The ubsan issue reported to us is with gcc:
https://github.com/harfbuzz/harfbuzz/issues/2953#issuecomment-823460893
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/100124] Why is "flexible array member '...' in an otherwise empty '...'" an issue?
2021-04-16 20:18 [Bug c/100124] New: Why is "flexible array member '...' in an otherwise empty '...'" an issue? gcc at behdad dot org
` (5 preceding siblings ...)
2021-04-20 19:32 ` gcc at behdad dot org
@ 2021-04-20 20:30 ` pinskia at gcc dot gnu.org
2021-04-20 22:16 ` gcc at behdad dot org
2021-04-26 22:26 ` msebor at gcc dot gnu.org
8 siblings, 0 replies; 10+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-04-20 20:30 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100124
--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Behdad Esfahbod from comment #6)
> > But I will note GCC treats (all) arrays at the end of a POD struct as a
> > flexible one so the question I have is more about the ubsan issue you are
> > running into, is that with GCC or with clang/LLVM?
>
> The ubsan issue reported to us is with gcc:
> https://github.com/harfbuzz/harfbuzz/issues/2953#issuecomment-823460893
Can you report that as a bug as GCC's rule is treat all arrays that end a POD
as a flexiable array? Please include the full preprocessed source that
produces the problem at runtime and the specific version of GCC that is used.
This is a GCC bug in mind due to this unless there is another field missing
that is in the source that you are not showing.
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/100124] Why is "flexible array member '...' in an otherwise empty '...'" an issue?
2021-04-16 20:18 [Bug c/100124] New: Why is "flexible array member '...' in an otherwise empty '...'" an issue? gcc at behdad dot org
` (6 preceding siblings ...)
2021-04-20 20:30 ` pinskia at gcc dot gnu.org
@ 2021-04-20 22:16 ` gcc at behdad dot org
2021-04-26 22:26 ` msebor at gcc dot gnu.org
8 siblings, 0 replies; 10+ messages in thread
From: gcc at behdad dot org @ 2021-04-20 22:16 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100124
--- Comment #8 from Behdad Esfahbod <gcc at behdad dot org> ---
(In reply to Andrew Pinski from comment #7)
>
> Can you report that as a bug as GCC's rule is treat all arrays that end a
> POD as a flexiable array? Please include the full preprocessed source that
> produces the problem at runtime and the specific version of GCC that is
> used. This is a GCC bug in mind due to this unless there is another field
> missing that is in the source that you are not showing.
Thanks. Will do after reproducing.
^ permalink raw reply [flat|nested] 10+ messages in thread
* [Bug c++/100124] Why is "flexible array member '...' in an otherwise empty '...'" an issue?
2021-04-16 20:18 [Bug c/100124] New: Why is "flexible array member '...' in an otherwise empty '...'" an issue? gcc at behdad dot org
` (7 preceding siblings ...)
2021-04-20 22:16 ` gcc at behdad dot org
@ 2021-04-26 22:26 ` msebor at gcc dot gnu.org
8 siblings, 0 replies; 10+ messages in thread
From: msebor at gcc dot gnu.org @ 2021-04-26 22:26 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100124
Martin Sebor <msebor at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution|--- |WONTFIX
--- Comment #9 from Martin Sebor <msebor at gcc dot gnu.org> ---
Thus resolving as WONTFIX.
^ permalink raw reply [flat|nested] 10+ messages in thread
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2021-04-16 20:50 ` [Bug c++/100124] " pinskia at gcc dot gnu.org
2021-04-16 21:02 ` jakub at gcc dot gnu.org
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